explicit call to __init__(self) in subclass needed?

[Andrew MacKeith]

I create a class like this in Python-2.6

 >>> class Y(str):
...   def __init__(self, s):
...      pass
...
 >>> y = Y('giraffe')
 >>> y
'giraffe'
 >>>

How does the base class (str) get initialized with the value passed to Y.__init__() ?

Is this behavior specific to the str type, or do base classes not need to be explicitly initialized?

Andrew